Lamps such as metal halide lamps commonly include a glass bulb (external bulb) with a concave portion formed at one end part, and an E-type base that is fixed to the one end of the glass bulb and has an eyelet part and a shell part.
A arc tube with electrodes disposed therein is provided inside the glass bulb.
Further, two leads for supplying electrical power to the electrodes extend out from the end of the glass bulb that is at the base.
One of the leads extends through a through hole provided in the eyelet part of the base, and is soldered to the outside surface of the eyelet part to form an electrical connection with the eyelet part.
The other lead extends from the concave section of the glass bulb, and passes between the glass bulb and the shell part of the base, being electrically connected to the shell part by solder poured into a gap between the concave portion and the base.
The solder poured into the gap between the concave portion and the base connects to the glass bulb and the base, and further functions to prevent the base from rotating with respect to the glass bulb.
Note here that, during metal halide lamp operation, the temperature of the base reaches 200° C. or more due to the heat generated in the arc tube. Hence, the solder used in the base section must not only be a high temperature solder, but one that does not melt even at temperatures of 200° C. or more.
Note that, in this specification, a solder whose solidus temperature is 183° C., the melting point of Pb-63Sn, or more is referred to as a high temperature solder.
Conventionally, solders such as Pb-20Sn and Pb-10Sn, which are described in JIS Z 3281 (1999), have been widely used as high temperature solders in the base section of lamps. Such high temperature solders, whose main constituent is Pb, have good solderability with respect to bases made from brass, nickel plated brass and the like.
Note that the “good solderability”referred to here means, in terms of the manufacturing process, that solder wire can be used, that solder is easily fed into the soldered section, and that the melted solder sets quickly; and in terms of solder quality, that respective alloy layers are formed between the lead material, the solder and the base material, and that the formed electrical connections are stable.
However, in recent years, due to environmental considerations, regulation against the use of high temperature solder with lead as the main constituent has continued globally. For this reason, there is a strong demand for lead-free solder, which does not include Pb.
To realize such a lead-free solder, solders containing Sn as their main constituent, and including other appropriately added elements have been examined.
Specifically, an alloy composed of mainly of Sn, containing between 1% and 2.5% exclusive of Cu by mass, has been proposed as a lead-free solder for use in lamps (see laid open Japanese patent application 2002-245974).
However, when lamps using this type of lead-free solder composed of an Sn—Cu alloy were used, for instance, in low temperature conditions where the temperature of the surroundings reached −40° C., a problem occurred. This problem was cracking in the soldered section, causing poor conduction between the leads and the base.
On consideration of this problem, the following conclusions could be drawn.
Namely, if a lamp is used in low temperature conditions where the ambient temperature is −40° C., when the lamp is not operating, the temperature of the base is the same as the temperature of the surroundings, which is −40° C. However, when operating, the temperature of the base is 200° C. or more. Hence, the temperature difference between the on and off states for the lamp is extremely large.
Moreover, if the lead free solder composed of Sn and Cu approaches or exceeds its solidus temperature, as is it does when the lamp is on, even supposing it does not melt, its bonding strength will be much reduced.
Due to the factors described above, it may be concluded that that, when a lamp is switched from on to off, the base temperature falls quickly from a high temperature of 200° C. or more to a very low temperature, the solder comes under a large stress because of the differing rates of thermal expansion for the solder and the base, and cracking occurs in the in the soldered section.
Further, in lamps using lead-free solder composed of Sn and Cu the problem occurred whereby the leads easily separated from the solder.
This problem is considered to occur because of insufficient heat-resistance in the lead-free solder.